1. Field of the Invention
The present invention is directed to wellbore drilling systems and, in certain aspects, to such systems with a percussion drill assembly which cyclically impacts a drill bit.
2. Description of Related Art
The rate of penetration of a formation by a drill bit is generally proportional to the weight, or downward thrust, placed on the drill bit. The addition of repetitive impact blows on a drill bit, e.g. those provided by a percussion drilling assembly, regardless of the weight applied to the bit, will increase the penetration rate of the drill bit. Due to the short duration of each impact blow, deviation of the borehole is minimized. Impact blows, therefore, can be used as a substitute for part of the weight on the drill bit.
One typical percussion drill assembly for drilling a borehole in an earth formation is described in U.S. Pat. No. 5,662,180 issued Sep. 2, 1997 incorporated fully herein by reference. The assembly disclosed in U.S. Pat. No. 5,662,180 has a compressor system with endless loop grooves formed in the outer surface of a rotary shaft whose rotation results in the cyclic compression of gas to provide cyclic impacts on a drill bit. This structure required special manufacturing techniques and equipment not available in all machine shops. High inertial loading on the contact surfaces of the endless loop grooves caused high wear, requiring frequent replacement of the complete rotary shaft section. Roller elements that extended into the endless loop grooves required specialized profile grinding and heat treatment processes, and were also subjected to the same high inertial loading that created the need for frequent replacement. The constraints of the compressor geometry in certain embodiments required some gas flow passages and gas volummes to be integral to the compressor piston, and the resulting complexity caused difficulty in achieving optimum performance. The length of the compressor assembly in certain embodiments made it difficult to stay within the required overall length constraints of the system.
There has long been a need, recognized by the present inventors, for an easily manufactured percussion drill assembly with relatively long-lived components and for relatively simple compressor systems for such assemblies. There has long been a need for such assemblies and compressor systems which are relatively more compact and which require relatively longer service intervals. There has long been a need, recognized by the present inventors, for a system which extends bit life in certain formations.